Horizon Split Ambient Occlusion

February 27, 2008 at 3:55 am (Game techniques, Presentations)

I have so much I want to write about what I saw at I3D and GDC that sitting down and doing it seems daunting. I will try and post my thoughts over the next few days.

One interesting poster at I3D (and also a short talk at GDC) was an extension to SSAO. The idea was to calculate a piece-wise linear approximation of the horizon ala horizon mapping. This is achieved by sampling the depth values along m steps ( they used 8 ) in n equally spaced directions (8 again) in the tangent frame. At each depth sample, you update the horizon value if the current depth sample is higher than the current estimation of the horizon in that direction. Sampling in this fashion reduces over-occlusion. This is actually very similar to the approximation to AO that Dachsbacher and Tatarchuk described in their poster at I3D last year, “Prism Parallax Occlusion Mapping with Accurate Silhouette Generation“. All of the authors from the poster are NVIDIA guys and they announced that they will release a whitepaper describing their method.

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2007: The year SSAO broke

February 10, 2008 at 4:39 pm (Demos, Game techniques)

kindernoiser

(image from RGBA demo kindernoiser)

One of my very first posts on this blog discussed the image space ambient occlusion paper by Shanmugam et al. That post has more hits than any other entry I’ve made here. So I thought I would take a few minutes and survey the state of the art in screen space ambient occlusion.

CryTek gave a presentation at SIGGRAPH 2007 in the Real-time Rendering course. They didn’t reveal any exact implementation details, but did reveal that they are only using the depth buffer from their z pre-pass (unlike the Shanmugam paper which uses depth and normals). They additionally noted that they use a per-pixel rotated disc of sample offsets to reduce sampling artifacts. This technique is common for reducing sampling artifacts in shadow mapping.

Iñigo Quilez ( of RGBA demoscene group, yay ) has created a website dedicated to his use of SSAO in the RGBA demo kindernoiser. One unique contribution is that he suggests doing a post-SSAO blur to lessen sampling artifacts. He has given the most detail by into the implementation of his technique, by far. A thread which follows some of the early results he was getting is @ gamedev. He also provides a analytic solution for the ambient occlusion due to a sphere here.

Megan Fox developed a technique which she refers to as “Crease shading“. It differs very little from the original Shanmugam technique. However, she provides an excellent breakdown of the technique and some comparison screenshots.

A tweak that I implemented for SSAO is to perform the ambient occlusion computation on a lower resolution texture and then upsample using an boundary respecting filter. I suggested on this blog (here) that the bilateral upsampling technique works well. I have received email from a few people that have tried it and are getting pretty good results.

So.. a whole lot of people were very interested in SSAO. Rightly so. Approximating an expensive technique like dynamic ambient occlusion (which itself is an approximation!) in a geometry-independent manner is a valuable tool. Of course, it has some serious drawbacks such as not being able to account for occlusion from back facing polygons and causing over occlusion, to name a few. If anybody knows of any other implementations/discussions of SSAO anywhere on the web, please post them here.

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